Neural mechanisms mediating inferential reasoning abilities in rats. The ability to make decisions in the face of uncertainty has been shown to decline disproportionately in aging adults. Because our lives are replete with ambiguous situations and little is known about the cognitive processes that normally mediate this ability, further research is needed to elucidate the underlying mechanisms. The present line of research uses rats to gain insight into these psychological and neurological processes. Specifically, the proposed research aims to investigate the interaction between prior experience and inferential reasoning in an ambiguous situation. Additionally, it will investigate the role of cholinergic modulation from the hippocampus-a structure particularly affected by age-related decline-in mediating these decision-making strategies. Rats will first be trained using instrumental conditioning to solve complex visual discriminations involving elements and cue compounds. In subsequent test sessions, we will occlude one of the visual cues with an opaque shield while presenting the other cue. Preliminary research from our lab suggests that rats are sensitive to the ambiguity created by the cover, but only under certain conditions. Specifically, this sensitiviy appears to be dependent on the rat's prior training history. The proposed experiments are designed to determine the critical features of prior experience that contribute to inferential reasoning processes in ambiguous situations. Features that will be explored include the influence of 1) cognitive effort required to successfully solve the initial discrimination by varyig the complexity of the task, 2) solution strategy engaged in the initial discrimination, and 3) the role of the neurotransmitter acetylcholine in the temporal lobe structure, hippocampus, in mediating sensitivity to the cover. The results of these experiments can aid in the development of interventions facilitating the use of these cognitive processes in both educational and clinical settings to remediate age-related cognitive decline.